Automotive vehicle body assemblies may involve, for example, the attachment of a formed galvanized steel sheet part to a hydroformed galvanized steel tubular part. A copper electrode for resistance spot welding is pressed against the sheet member of the tube/sheet assembly at an intended weld site with a copper backup electrode supporting the opposite side of the tube member. In a production operation, several spot welds in a pattern may be required to suitably attach the sheet to the tube. And in a vehicle body assembly, several sheets may be welded to several tubes. Such operations now typically involve a digitally controlled welding machine setup in which robot arms, for example, carry welding guns comprising a resistance spot welding electrode and an opposing backup electrode and are moved around an assembly to form the welds. At each sheet/tube weld location the welding gun clamps on the galvanized surfaces for good electrical contact. Pneumatic pressure is usually supplied to the gun for this clamping force. A high amperage welding current is momentarily passed between the copper electrodes through the sheet layer and around the tube at the weld location to briefly fuse the tube/sheet interface to form a weld nugget.
Steel is a material of choice for sheet and tube body structures because of its formability and weldability. And preferably the steel is galvanized for corrosion resistance. Current materials are quite strong and relatively thin sheets and tubes can be used in these applications. However, it has proven difficult to produce good spot welds between such sheets and tubes by the described process on galvanized steel because cracks form at the weld sites, particularly in the sheet above the weld nugget.
The cracks are primarily caused by localized high tensile stress at the weld joint and are exacerbated by the presence of the zinc element in the galvanized steel. Because the tube is typically a relatively thin-walled member, when the upper electrode presses against the sheet and tube, the tube and sheet tend to flex. Such flexure of the sheet and tube results in the high tensile stress at the weld joint and, thus, presents a unique problem in the art of spot welding. Also, during the welding cycle, the zinc element enters into the grain boundaries of iron within the galvanized steel and remains in a liquid state at a temperature well below the freezing or solidifying point of the steel. After the heating cycle, but before the electrode force is released, the liquid zinc possesses little strength and, consequently, provides little resistance to cracking. As a result it has been difficult to spot weld sheet-to-tube galvanized steel assemblies.
It is an object of this invention to provide a resistance spot welding method for joining galvanized steel sheets to galvanized steel tubes.